Taking into account the information you have shared with me, go to the query editor and filter out all the null.
When the table is transformed into the final version it automatically sorts the value by the first column so the null come in first so it appear has the table is all null since it only uses the top N to do the relationships preview.
So if you filter out all the null from one of the columns that you now must have values the lines that aren't to be consider will not be uploaded and your model will work properly.
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